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Sulfides: the good, the bad and the ugly

Winemaking (Photo: How Stuff Works website)

Gone are the days where squeaky clean wines were the norm. We now see a whole spectrum of styles encompassing the full gamut of flavours and aromas, which is a good thing.

Experimentation is occurring industry wide, despite the hip small players getting most of the attention. As curiosity is part of human nature, you will find that in most cellars, large and small, there will be a few experimental barrels hiding somewhere.

Natural winemaking, extended lees ageing, wild yeasts, pre-fermentation skin contact, co-fermentation, whole bunch fermentation, post-ferment maceration — the techniques are numerous and varied, not to mention vineyard trials. Wine producers strive to find their own voice and have different goals. Some chase vineyard expression and varietal integrity. For others, it’s complexity and texture. And then there are those seeking individuality and uniqueness, in whatever form it comes.

I like what the avant-garde wine movement is doing for the wider wine industry. It has prompted some more conservative producers to loosen the reins a little and be more experimental. And I think many wines are better for it. I greatly applaud and appreciate the increased diversity in style that has come from this approach.

While I have the joy of tasting many impressive and stimulating wines, including ones with complex and interesting sulfide compounds, I also see wines at the opposite end of the spectrum. And the economic reality for most producers is that these wines still need to be sold. Sometimes unpleasant characters only reveal themselves after a wine hits the market, which can be a big problem for wineries and consumers alike.

Sulfides can be the worst offenders in this case. When their formation is unplanned and adverse, they have the potential to mask every other element that the winemaking and viticultural team have worked so hard to achieve.

I recently tasted a very expensive red that had an almost pure odour of asafoetida. For those familiar with Indian cooking, asafoetida adds depth to dishes due to its onion-like flavour, though it has the most atrocious smell. Descriptors of ‘rotting feet’ or ‘devil’s sweat’ give you an idea of its acrid nature.

It was the first time, in twenty-five years of tasting wine, that I had ever seen this specific character. It reminded me of the complex, and somewhat unpredictable, world of sulfide chemistry. Here is a broad and brief overview of some of the types of unpleasant sulfides than can be present in bottled wine.

Hydrogen sulfide

Hydrogen sulfide (H2S) is also known as rotten egg gas. Different yeasts produce varying amounts, linked to genetics, stress and the amount of available sulfur in the juice. However, it is commonly produced when yeasts don’t have enough nitrogen during fermentation. It is quite an unstable compound and exposure to air is usually sufficient for it to blow away. So, if you open a bottle that has H2S, decanting (or even just a vigorous shake the glass) is normally enough to remove it.

Mercaptans (thiols)

The term mercaptan is a broad term for a group of compounds which contain a thiol group (made up of sulfur and hydrogen). If H2S is not removed during the winemaking process, it can react with other molecules to form mercaptans, though there are different ways these compounds can be formed. At very low levels mercaptans can add complexity, such as a struck match character. However, if you get high levels of certain mercaptans, they can express themselves as a rotten vegetable character which is distinctly unpleasant. If you find these characters in your bottled wine, they will not blow off. However, they can look less intense after decanting, once the fruit has opened.

Disulfides

Once mercaptans form, there is a possibility that they will oxidise to disulfides, which can be distinctly unpleasant, with a range of smells including onion and rubber.

So as for the asafoetida taint, how did it arise? I suspect it is a type of disulfide, though I certainly don’t claim to be an expert this area. Perhaps it has something to do with how the wine was prepared before bottling? Or the amount of oxygen it was exposed to during production? Or maybe it has something to do with trace metals present in the wine? It is a very complex area; one to learn more about. But, let’s hope it is the first and last time I see it.

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